The human T-cell lymphoma virus, HTLV-I, has been found to be associated with patients with adult T-cell leukemia. Studies are underway to understand the mechanism of malignant transformation and immunologic response of individuals infected with this virus. An HLA class I epitope is expressed when cells are infected with HTLV-I but not HTLV-II. Differences in the env region of the provirus suggest that this epitope is present in the large envelope protein of HTLV-I. HTLV-I virus has been found in B-cell lines that grow spontaneously from patients with adult T-cell leukemia. The virus is expressed and infects T-cells indicating selective tropism for T-cells and not B-cells. These B-cell lines spontaneously produce acid labile and acid stable beta interferon and B-cell growth factor. T-cell clones with specific function lose their specificity when infected with HTLV-I. Immunoglobulin from B-cells from chronic lymphocytic leukemia cells from patients with serum antibodies to HTLV-I have antibody activity to HTLV-I glycoproteins suggesting an indirect effect of the virus in diseases other than adult T-cell leukemia. Antibodies were raised to HTLV-I protein produced by transfection of mouse cells with a vector containing selected portions of two retroviruses. This antibody reacts with the small envelope protein (p21) and p42 the product of the pX region of the virus.